1. Field of the Invention
The invention relates in general to circuits for operation of lighting means, for example of gas discharge lamps, or inorganic or organic LEDs.
2. Related Technology
In many operating devices for lighting means, it is normal practice for the lighting means either to be operated at a high frequency (in particular in the case of gas discharge lamps) or else by means of pulse-width modulation (for example in particular in the case of LEDs). So-called driver circuits are provided for this purpose which, for example, may be designed in the form of a pulse-width modulator, a full-wave bridge circuit or a half-wave bridge circuit, etc. These driver circuits are normally supplied with a DC supply voltage on which a so-called ripple may be superimposed.
A so-called PFC circuit (Power Factor Correction) is often used as an active power factor correction circuit for provision of this DC supply voltage. This PFC circuit therefore produces the DC supply voltage on the basis of a normally rectified mains supply voltage.
The present invention in this case always relates to so-called actively clocked PFC circuits, that is to say circuits in which the power consumption of the PFC circuit is predetermined by clocking a switching element, such as a transistor.
When the switching element is closed, an inductance is in this case magnetized, which is then discharged (demagnetized) into an energy storage capacitor via a diode when the switch is opened. By way of example, the PFC circuit may be a step-up converter, a flyback converter or else an isolated flyback converter (in this case, the inductance has a secondary winding, via which the inductance is demagnetized via a diode into an energy storage capacitor).
A specific problem occurs when the power consumption of the load circuit which is connected to the output of the PFC circuit (this load circuit having the driver circuit and the lighting means) can vary to a very major extent, as is the case in particular when the connected lighting means are dimmed to very low dimming levels (for example 1% or less of the rating). LEDs in particular can be dimmed to a range well below 1% of the rating).
During this severely dimmed operation, the PFC circuit changes automatically and in an uncontrolled manner according to the prior art to a so-called burst mode, in which the circuit is switched on and off at a relatively low frequency, in order to avoid an impermissible excessive rise in the output voltage of the PFC circuit. This burst mode (switching on and off) has a very low frequency and in consequence causes various problems, such as instability of the DC supply voltage for the connected driver circuit, acoustic problems, and possibly flickering of the lighting means, etc.